Scientific Abstract Although a 10% weight loss significantly reduces the risk of obesity-associated diabetes and heart disease, fewer than 20% of individuals achieve and maintain this weight loss. Thus, obesity underlies a large portion of health care costs in the United States. Consumption of palatable foods above homeostatic needs (reward-based feeding) is a likely contributor to weight regain. Ventral tegmental area (VTA) dopamine neurons play a key role in reward-based feeding. Metabolic state influences reward circuitry. However the mechanisms underlying this regulation are unclear. The lateral hypothalamic area (LHA) orexin glucose-inhibited (GI) neurons which provide excitatory input to the VTA dopamine neurons may be an important link between metabolic status and reward-based feeding. For example, orexin mediates the fasting-induced increase in reward-based feeding. Our preliminary data show that fasting also reduces the inhibitory effect of glucose on LHA orexin-GI neurons leading to increased activation in low glucose. This proposal tests the hypothesis that weight loss increases activation of LHA orexin-GI neurons by glucose deficit. This change in glucose sensitivity reinforces reward-based feeding by causing persistent changes in glutamate signaling onto the VTA dopamine neurons. As a result pre-prandial glucose decreases could enhance intake of the subsequent meal. Three Specific Aims will test this hypothesis in vitro and in vivo: 1) Determine whether weight loss enhances the response of orexin GI neurons to decreased glucose; 2) Determine whether weight loss enhances synaptic plasticity in VTA dopamine neurons in a glucose and orexin dependent manner; 3) Determine whether increased LHA glucose level attenuates the effects of fasting and weight loss on reward based behavior. These studies will increase understanding of the mechanisms underlying difficulties in achieving and maintaining weight loss.